Audio crossover
From Wikipedia, the free encyclopedia
Audio crossovers are a class of electronic filters designed specifically for use in audio applications, especially hi-fi. A commonly used dynamic loudspeaker driver is incapable of covering the entire audio spectrum all by itself. Thus, crossovers serve the purpose of splitting the audio signal into separate frequency bands which can be handled by individual loudspeaker drivers optimized for those bands. A combination of multiple drivers each catering to a different frequency band constitutes most hi-fi speaker systems. An audio crossover may also be constructed mechanically and is commonly found in full-range speakers.
Another use of crossovers is multiband processing, where the audio signal is split up into bands, which can be adjusted separately. After the adjustments, the individual bands are mixed together again. Some examples are: multiband dynamics (compression, limiting, de-essing), multiband distortion, bass enhancement, high frequency exciters, noise reduction (for example: Dolby A noise reduction).
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[edit] Overview
An ideal audio crossover would split the incoming audio signal into separate bands that do not overlap at all, and which when added together would give the signal unchanged in both frequency and phase response. While this is not achievable in practice, this behavior can be approximated. Many different crossover types are used, but they generally fall under one of the classifications provided below.
[edit] Crossover classification
[edit] Classification based on the number of filter sections
In loudspeaker specifications, one often sees a speaker classified as an "N-way" speaker. N is a positive whole number greater than 1, and it indicates the number of filter sections. A 2-way crossover consists of a low-pass and a high-pass filter. A 3-way crossover is constructed as a combination of low-pass, band-pass and high-pass filters (LPF, BPF and HPF respectively). The BPF section is in turn a combination of HPF and LPF sections. 4 (or more) way crossovers are not very common in speaker design, primarily due to the complexity involved, and the cost/complexity is not generally justified by better acoustic performance.
Recently, a number of speakers advertise what are called "N.5-way" crossovers. This usually indicates the addition of an extra bass driver with a crossover designed such that it augments the bass response or compensates for diffraction loss.
Remark: Filter sections mentioned here is not to be confused with the individual 2-pole filter sections that a higher order filter consists of.
[edit] Classification based on components
Crossovers can also be classified based on the type of components they use.
[edit] Passive crossovers
A passive crossover is made entirely of passive filters. Passive filters employ passive components like resistors, capacitors and inductors for their operation. These are relatively inexpensive, easy to adjust by unwinding inductors and/or paralleling capacitors, and add no noise. On the negative side,
- These tend to be bulky and always cause some power loss.
- They suffer from dependence on the individual speaker drivers' input impedance.
- If the amplifier overloads, any distortion products above the crossover frequency will be sent to the tweeter.
- Any resistance in components between the speaker and amplifier will reduce damping and change driver response if not compensated for in the design.
[edit] Active crossovers
An active crossover contains active components in its filters. The most commonly used active device is an op-amp and active crossovers are operated at the levels meant to drive amplifiers in contrast to passive crossovers which operate at the high-levels meant for speakers. Active crossovers always require the use of power amplifiers for each band. Thus a 2-way active crossover needs two amplifiers — one each for the woofer and tweeter. This means that an active crossover based system might end up costing more than a passive crossover based system. It also requires the use of a tweeter protection capacitor (≥ 22 µF) since the tweeter is now directly connected to the amplifier and may be damaged due to DC or the short thump an amplifier produces as the amplifier is powered on. The cost disadvantage is offset by the following gains:
- Frequency response independent of the speaker drivers input impedance.
- Ability to easily vary or fine tune each frequency band for the particular speaker drivers being used.
- Complete isolation of the drivers — the tweeter will never know if the woofer is being over-driven.
- The power amplifiers are directly connected to the speaker drivers, thereby maximising the damping effect and potentially improving the transient response of the system.
[edit] Mechanical crossovers
This crossover is entirely mechanical in design and uses the properties of the materials used in a speaker to achieve filtering. It is commonly found in full-range speakers which are designed to cover as much of the audio band as possible. A mechanical crossover is constructed by coupling the diaphragm of the speaker to the voice coil through a compliant section and directly coupling a small light-weight cone called whizzer to the voice coil. The compliant material ensures that the diaphragm responds only to lower frequencies while the whizzer which is directly coupled to the coil can respond to the rapid movements of the coil at high frequencies. This combination results in the diaphragm having an upper cut-off frequency while the characteristics of the whizzer and voice coil set the lower limit to the whizzer's response, thereby approximating a crossover action. The choice/weight of materials used for the diaphragm, whizzer and the speaker's suspension determine the crossover frequency and the accuracy of the crossover. This crossover is much more complex to design, especially if the highest degree of performance is desired. A degree of trial and error is required. This type of crossover (rather the speaker that uses it) can considerably simplify the overall setup since it eliminates the need for an electrical crossover, at least as far as the range of frequencies this speaker is designed to handle is concerned. Also, due to the way this crossover is fabricated, speakers using it are almost always 2-way. Those who do not prefer the sound of fullranges might argue that the act of making a single diaphragm respond separately to low and high frequencies dooms it to do neither justice. See full-range speaker for construction details.
[edit] Digital Crossovers
Crossovers can be implemented digitally using a DSP chip or a microprocessor. They either use the digital approximations of traditional analog IIR filters (Bessel, Butterworth, Linkwitz-Riley etc.), having similar character to the analog versions, or they use Finite impulse response (FIR) filters. FIR filters can be constructed easily using DSP chips or microprocessors. They usually have a higher order, but their behaviour is different. They can be designed in a way that they have a linear phase response, which is desirable for crossovers. As a result, they are often used as crossovers in digital signal processing.
[edit] Classification based on filter order or slope
Just as filters have different orders, so do crossovers — depending on the filter slope they use. The final acoustic slope may be completely determined by the electrical filter or may be achieved by combining the electrical filter's slope with the natural characteristics of the driver. In the former case, the only requirement is that each driver has a flat response at least to the point where its filter section is approximately -10dB. In the latter case, the final acoustic slope is usually steeper than of the electrical filters used. A third or fourth order acoustic crossover often has just a 2nd order electrical filter. This however requires that speaker drivers have a well behaved frequency response in the vicinity of the crossover frequency. In the discussion below, the characteristics of the electrical filter order is discussed, followed by a discussion of crossovers having that acoustic slope and their advantages or disadvantages.
Most audio crossovers use first to fourth order electrical filters. Higher orders are not generally implemented passively in loudspeakers, but are sometimes found in electronic equipment under circumstances where their cost and complexity is justified.
[edit] First order crossovers
1st order filters have a 20 dB/decade (or 6 dB/octave) slope. All 1st order filters are classified as Butterworth filters. 1st order filters are considered by many audiophiles to be ideal for crossovers. This is because this filter type is transient perfect, meaning it passes both frequency and phase unchanged. It also uses the fewest parts and has the lowest insertion loss (if passive). A 1st order crossover tends to allow relatively more unwanted frequencies to get through in the LPF and HPF sections. While woofers can easily take this, the tweeters may be damaged since they are not designed to handle large powers and lower frequencies.
In practice, loudspeakers with true first order acoustic slopes are difficult to design because they require large overlapping driver bandwidths and the shallow slopes mean that non-coincident drivers interfere over a large range of frequencies and cause large response shifts off-axis.
[edit] Second order crossovers
2nd order filters have a 40 dB/decade (or 12 dB/octave) slope. 2nd order filters can have a Bessel, Linkwitz-Riley or Butterworth characteristic depending on design and choice of components used. This order is commonly used in passive crossovers as it offers a good balance between complexity, response and tweeter protection. When designed with time alignment, these crossovers have a symmetrical polar response, as do all even order crossovers.
It is commonly thought that there will always be a phase difference of 180° between the outputs of a (second order) low-pass filter and a high-pass filter having the same crossover frequency and hence in a 2-way system, the high-pass is usually inverted to correct for phase. For passive systems the tweeter is wired opposite to the woofer and for active systems the high-pass filter's output is inverted. In 3-way systems the mid-range driver or filter is inverted. This is generally only true when the speakers have a wide response overlap and the acoustic centers are aligned.
[edit] Third order crossovers
3rd order filters have a 60 dB/decade (or 18 dB/octave) slope. These crossovers are usually of the Butterworth filter characteristic and phase response is very good, the sum being flat and in phase quadrature, similar to a first order crossover. The polar response is asymmetric. In the original D'Appolito MTM arrangement, a symmetrical arrangement of drivers is used to create a symmetrical off axis response when using 3rd order crossovers.
Third order acoustic crossovers are often achieved with a first or second order filter circuit.
[edit] Fourth order crossovers
4th order filters have an 80 dB/decade (or 24 dB/octave) slope. These filters are complex to design in passive form. A 4th order crossover with −6 dB crossover point and flat summing is also known as a Linkwitz-Riley crossover (named after the inventors of this crossover). It can be constructed in active form by cascading two 2nd order Butterworth filter sections. The output signals of this crossover order are in phase, thus not requiring any inversion unless the driver acoustic centers are not aligned.
[edit] Higher order crossovers
Passive crossovers giving acoustic slopes higher than 4th order are not common, because of their cost and complexity. They are sometimes used in active crossover modules.
[edit] Mixed order crossovers
Crossovers can also be constructed with mixed order filters, for example a second order lowpass and third order highpass. These are generally passive and are used for several reasons, often when the component values are reached by optimization. A higher order tweeter crossover can sometimes help to compensate for the time offset between the woofer and tweeter, caused by non coplanar acoustic centers.
[edit] Classification based on topology
[edit] Parallel crossovers
These are by far the most common. Electrically the filters are in parallel and thus the various filter sections do not interact. This makes them easier and more predictable to design because the sections can be considered separately.
[edit] Series crossovers
Crossovers using this topology are almost always passive because it is easiest to construct in passive form. In this topology, the individual filters are connected in series, with a driver or driver combination connected in parallel to each filter. As can be seen in the image, a low-pass filter in shunt with the tweeter results in a high-pass response for the tweeter, since the lower frequencies are shunted by the LPF via the woofer. Similarly, the HPF in parallel with the woofer shunts away the higher frequencies via the tweeter - a low-pass response for the woofer. One advantage (or disadvantage, depending on how one looks at it) of this crossover, is that the crossover sections interact with each other. A change in any component affects both highpass and lowpass sections. To some extent, this makes the crossover somewhat self-balancing - the crossover frequency changes, but the system still sums substantially flat. This characteristic makes them appealing to designers who are not using measuring equipment or software.
[edit] References
- DIY Audio at lalena.com - DIY Audio site with information on building crossovers. Includes a crossover calculator for 15 different types of crossovers.
- Article about active crossovers
- Comparison of active and passive crossovers
- Comparison of series and parallel crossovers
- Description of a L-R crossover
- Passive crossover design article
- ESP web-site
- Linkwitz Lab Crossovers
- Linkwitz-Riley Crossovers: A Primer
- Design of Digital Linear Phase FIR Crossover Systems
- Full-range Driver & Loudspeaker Theory
- http://www.audioholics.com/productreviews/loudspeakers/ThielSS2subwoofer.html
- Audioholics.com Filter & Crossover Types
- A Bessel Filter Crossover, and Its Relation to Others
- Active Lowpass Filter Design
- Second Order Low-pass Filter Responses
